The invention relates to force sensing apparatus and in particular, force sensing apparatus for monitoring the bond force in an ultrasonic welding machine.
Ultrasonic welding is commonly used for wire bonding when packaging semiconductor devices and for other bonding operations. Force sensors are used in the ultrasonic wire bonding machines to either monitor the quality of the wire bond by measuring the bond force or to generate the required bond force in response to signals from the sensor.
In some arrangements the force sensor is located in a structure which is external from and isolated from the transducer. In other designs, the force sensor is mounted on the transducer body itself.
Sensors residing on the body of the ultrasonic transducer are normally of the balanced strain gauge design. They measure the surface strains produced by bending of the transducer during bonding and thus indirectly obtain information regarding the bond force. However, as they contact the transducer body they exert a load on the transducer, which interferes with the operation of the transducer.
Force sensors located in force arms external to the bond-head are, in general, not compact.
In general, piezoelectric force sensors and electrorestrictive force sensors suffer from the problem of output voltage drift. This introduces errors in successive measurements. To overcome this problem, a number of the prior art machines (see for example, U.S. Pat. Nos. 4,903,883 and 5,607,096) use two or more sensors to mutually compensate for any drift.
However, this has the disadvantage of increasing the cost and size of the sensor arrangement and also increases the processing required for the sensor signals.
In accordance with a first aspect of the present invention, there is provided force sensing apparatus for sensing deflection of a transducer in an ultrasonic welding machine, the apparatus comprising a body member, a transducer holder adapted to hold an ultrasonic transducer, the transducer holder being fixed to the body member, and a force sensor located between adjacent surfaces of the body member and the transducer holder to sense a force applied between the surfaces.
Preferably, the force sensor senses a force applied in a direction substantially parallel to the longitudinal axis of the transducer.
Typically, the adjacent surfaces of the body member and the transducer holder are oblique to the longitudinal axis of the transducer. Preferably, the adjacent surfaces are substantially perpendicular to the longitudinal axis of the transducer.
Preferably, the force sensor comprises a piezoelectric force sensor. Alternatively, the force sensor may comprise an electrostrictive force sensor.
Preferably, the sensing apparatus comprises a biasing device located between one of the adjacent surfaces and the force sensor to provide a preload to the force sensor.
In accordance with a second aspect of the present invention, there is provided a method of compensating a force sensor for performance changes in the sensor, the method comprising:
(a) at the beginning of a first time interval taking a first reference reading from the sensor with a reference force applied to the sensor;
(b) using the first reference reading as a reference for readings from the sensor during the first time interval to obtain an indication of an unknown force applied to the sensor; and
after the end of the first time interval repeating steps (a) and (b).
An advantage of this aspect of the invention is that it permits force sensors which are prone to drift by resetting the reference level for the sensor at periodic time intervals.
Preferably, the force sensor may be a piezoelectric force sensor or a electrorestrictive force sensor.
Typically, the reference reading is a reading of the voltage output from the sensor.
Typically, steps (a) and (b) are repeated as frequently as necessary to obtain a sufficiently accurate indication of the unknown force for the application in which the force sensor is being used. The length of subsequent time intervals may be the same as or different from the first time interval.
Typically, the force sensor may be a force sensor for monitoring the contact force of an ultrasonic welding transducer on a workpiece. For example, the transducer may be a wire bonding transducer and the workpiece may be a contact pad of a semiconductor die or lead frame to which a wire is to be bonded by the transducer.
Preferably, where the force sensor monitors the contact force of an ultrasonic weld transducer, the first time interval corresponds to the time interval between separate welds, or between a number of welds.